Create walls from a plane?
In edge select mode (CtrlTab&gt;Edge), select all the outer edges and press EZ to extrude along the Z axis:

In the above gif I used the new boundary edge selection tool, which is currently only available in the latest development builds (it will be in 2.71).
If you are using an older version, you can select the boundary edges like so:

Enter edit mode (Tab)

Select all (A)

Press 3D view &gt; Header &gt; Select &gt; Select Boundary Loop: